1. Field of the Invention
The present invention relates to a reticle for use in exposing a semiconductor, a method of producing the reticle, and a semiconductor device, and more particularly, to a reticle to be used in exposure for producing a resist pattern, a method of producing the reticle, and a semiconductor device.
2. Description of Related Art
A photolithography process for manufacturing a semiconductor device comprises a photoresist application process, an exposure process, and a development process. Of these processes, the exposure process is a process for faithfully reproducing an integrated circuit pattern formed on a reticle onto a photoresist pattern formed on a wafer, through use of a stepper. An optical reduction-projection exposure system is widely used as an exposure system.
FIG. 4 shows an optical reduction-projection exposure system 100 (hereinafter referred to as a xe2x80x9cstepperxe2x80x9d) using a conventional reticle 110. In FIG. 4, reference numeral 101 designates alight source; 102 designates a shutter for adjusting light emitted from the light source 101; 103 designates a beam-shaping optical system for shaping the light emitted from the light source 101 by way of the shutter 102 so as to assume a desired geometry; 104 and 105 designate Fly""s eye lenses for uniformly illuminating the area to be exposed; 106 designates a vibration mirror for guiding the light emitted from the Fly""s eye lens 104 to the Fly""s eye lens 105; 107 designates a reticle blind for covering an area on a reticle 110 other than the area onto which a circuit pattern is to be projected through exposure; 109 designates a condenser lens for illuminating the entire surface of the reticle 110; 108 designates a mirror for guiding to the condenser lens 109 the light which has passed through the reticle blind 107; 110 designates a reticle on which is formed a circuit pattern to be projected onto the wafer 112; 111 designates a reduction-projection lens for projecting, in a reduced manner and onto the wafer 112 the light which has passed through the reticle 110; and 112 designates a wafer on which a circuit pattern formed on the reticle 110 is patterned through exposure.
FIG. 5 shows diffraction of light caused when the reticle 110 is exposed to coherent light formed from a plane wave of coherent wavelength and phase. In FIG. 5, the same reference numerals as those provided in FIG. 4 designate the same elements, and hence their explanations are omitted here. In general, as shown in FIG. 5, in a case where the reticle 110 is exposed to coherent light, light 50 which has entered the reticle 110 at right angles is divided into light 52 (0-order light) which travels straight ahead and light beams 51 and 53 (xc2x1m-order light beams, where m=1, 2, 3, . . . ) which are diffracted, by the surface (lower surface) of the rectile 110. Here, provided that an angle of diffraction; for example, an angle of diffraction exemplified by an angle formed between the straightly-traveling light 52 and the diffracted light 51 is xcex8; a pattern pitchxe2x80x94which is in the proportion of one line representing the width of linear patterns formed on the rectile 110 to one space representing the space between the linesxe2x80x94is taken as P; an index of refraction of the reticle 110 is taken as xe2x80x9cnxe2x80x9d; the numeric aperture expressed as NA=nsinxcex8 is taken as NA; and the wavelength of the incident light 50 is taken as xcex, there stands a relationship between the pattern pitch P and the numeric number PA, as expressed below.
P=mxcex/NAxe2x80x83xe2x80x83(1)
As represented by Eq. (1), the numeric aperture NA and the angle of diffraction xcex8 increases with a reduction in the pattern pitch P. In contrast, if the pattern pitch P is constant, diffracted light of greater order xe2x80x9cmxe2x80x9d has a greater numeric aperture NA, and the diffraction angle xcex8 increases. As mentioned above, Eq. (1) represents the minimum pattern pitch P at which m-order light can be collected at the predetermined wavelength xcex and the numeric aperture NA. For instance, when the wavelength xcex is 248 nm and the numeric aperture NA is 0.55, the minimum pattern pitch P at which light of m=xc2x11 order can be collected can be expressed as P=1xc3x97248 (nm)/0.55=451 nm=0.45 xcexcm. In the case of a circuit pattern whose width is smaller than 0.45 xcexcm, diffracted light of m=xc2x11 order cannot be collected. If only 0-order light is used for exposure, an image patterned on the wafer 112 loses contrast and is not resolved. Accordingly, as a circuit pattern formed on the reticle 110 becomes more minute, the contrast of an image of the circuit pattern projected on the wafer 112 through exposure is reduced. If the circuit pattern becomes smaller than a certain size, the circuit pattern will not be resolved, thereby hindering formation of the circuit pattern (e.g., a resist pattern) onto the wafer 112.
Conventionally, to solve the foregoing problem, the contrast of an image to be resolved on the wafer 112 is increased through use of modified illumination, thereby resolving a more minute resist pattern.
FIGS. 6A and 6B are descriptive views of modified illumination, wherein FIG. 6A shows exposure without use of modified illumination and FIG. 6B shows exposure using modified illumination. In FIG. 6, those reference numerals which are the same as those provided in FIGS. 4 and 5 designate the same elements, and hence repetition of their explanations is omitted here. In FIG. 6A, reference numeral 66 designates a circuit pattern formed on the surface (lower surface) of the reticle 110; 64 designates a depth of focus of the light resolved on the wafer 112; and 65 designates the contrast of light resolved.
Modified illumination refers to an illumination technique for causing a luminous flux to which a reticle is to be exposed to enter an optical system obliquely, through use of a diaphragm provided outside the optical axis of the optical system. FIG. 6B shows a case where a luminous flux 60 of exposing radiation is caused to enter the reticle 110 obliquely through use of modified illumination. In FIG. 6B, reference numeral 62 designates 0-order light diffracted by the reticle 110; 61 designates +1-order light diffracted by the reticle 110; 63 designates xe2x88x921-order light diffracted by the reticle 110; 67 designates the depth of focus (DOF) of the light resolved on the wafer 112; and 68 designates the contrast of the light resolved on the wafer 112. A comparison between exposure without use of modified illumination and exposure using modified illumination reveals that the focal depth DOF 67 is greater than the focal depth DOF 64, and that the contrast 68 is greater than the contrast 65. The contrast of the image formed on the wafer 112 can be improved by means of increasing the luminous flux of exposing radiation 60 that enters obliquely, through use of modified illumination. Consequently, a more minute resist pattern formed on the reticle 110 can be resolved.
FIGS. 7A and 7B show a case where the previously-described modified illumination is applied to a stepper 100 shown in FIG. 4, wherein FIG. 7A shows exposure without use of modified illumination and FIG. 7B shows exposure using modified illumination. In FIGS. 7A and 7B, those reference numerals which are the same as those provided in FIGS. 4 through 6 designate the same elements, and hence repetition of their explanations is omitted here. In FIG. 7A, reference numeral 109 designates a condenser lens corresponding to the previously-described condenser lens; and 40 designates the vertical distance (positive and negative) over which the stepper 100 is moved with reference to a horizontal position 41 of the reduction-projection lens 111, which is taken as 0. In FIG. 7B, reference numeral 74 designates a diaphragm plate in which a diaphragm located outside the optical axis 50 of the optical system is inserted; 76 designates a luminous flux which is emitted from the light source 101 and enters the condenser lens 109 by way of the diaphragm plate 74; and 60 designates a luminous flux which enters the reticle 110 obliquely, as has been described previously. The diaphragm 74 may be provided at the back of the Fly""s eye lens 105.
As mentioned previously, oblique incident light 60 which is inclined at a certain angle with respect to the reticle 110 forms an image by means of +1-order light 61 and 0-order light 62 or by means of 0-order light 62 and xe2x88x921-order light 63. For example, provided that the incident light 60 has a wavelength xcex of 248 nm and an numeric aperture NA of 0.55, the minimum pattern pitch P capable of collecting the 0-order light 62 and the +1-order light 61 or the 0-order light 62 and the xe2x88x921-order light 63 is defined as P=0.45/2=0.225 xcexcm, because the pattern pitch P merges the 0-order light and 1-order light into a single light ray. Consequently, when compared with the contrast of an image formed through exposure to only the light 50 that falls on the reticle 110 at right angles, the contrast of the image formed on the wafer 112 is increased further, hence enabling resolution of a minute resist pattern.
FIG. 8 shows details of a normal mask which has conventionally been employed for exposure through use of the previously-described modified illumination; for example, the 6-inch reticle 110. In FIG. 8, reference numeral 80 designates a flat surface of the reticle 110 on which the luminous flux of exposing radiation 60 falls and which is formed from quartz 86. Reference numeral 82 designates a pattern surface formed of a pellicle; and 88 designates a layer which is made of Cr or CrOx and is sandwiched between the glass surface 80 and the pattern surface 82.
As has been described above, the glass surface 80 of the conventional reticle 110 is flat. Consequently, in the case where modified illumination is used for exposure, there arises a problem of a necessity for modifying the geometry of the diaphragm plate 74 as a circuit pattern to be formed on the reticle 110 becomes more minute.
The present invention has been conceived to solve the above-described problem, and the object of the present invention is to provide a reticle for use in exposing a semiconductor capable of enhancing the contrast of a resist pattern to be patterned on a wafer through exposure, without involvement of a modification in the geometry of a diaphragm even when a circuit pattern to be formed on a reticle becomes more minute, as well as to provide a method of manufacturing the reticle and a semiconductor device.
According to a first aspect of the present invention, there is provided a reticle for use in exposing a semiconductor, comprising: a transmission section having a transmissivity of about 1 with respect to illumination light for exposing, and whose entrance-side surface facing the illumination light has uniform irregularities; and a semi-transmission section imparting to the illumination light passing therethrough a phase difference of about (2n+1)xcfx80(xe2x80x9cnxe2x80x9d is an integer, and xcfx80 is the ratio of the circumference of a circle to its diameter), which is formed on an lower surface opposite to the entrance-side surface of the transmission section, and whose transmissivity with respect to the illumination light having passed through the transmission section is one-forth or less that of the transmission section.
According to a second aspect of the present invention, there is provided a method of producing a reticle for use in exposing a semiconductor, comprising the steps of: forming a transmission section having a transmissivity of about 1 with respect to illumination light for exposing, and whose entrance-side surface facing the illumination light has uniformly irregularities; and forming a semi-transmission section imparting to the illumination light passing therethrough a phase difference of about (2n+1)xcfx80(xe2x80x9cnxe2x80x9d is an integer, and xcfx80 is the ratio of the circumference of a circle to its diameter), which is formed on a lower surface opposite to the entrance-side surface of the transmission section, and whose transmissivity with respect to the illumination light having passed through the transmission section is one-forth or less that of the transmission section.
According to a third aspect of the present invention, there is provided a semiconductor device manufactured through use of a reticle, the reticle comprising: a transmission section having a transmissivity of about 1 with respect to illumination light for exposing, and whose entrance-side surface facing the illumination light has uniform irregularities; and a semi-transmission section imparting to the illumination light passing therethrough a phase difference of about (2n+1)xcfx80(xe2x80x9cnxe2x80x9d is an integer, and xcfx80 is the ratio of the circumference of a circle to its diameter), which is formed on an lower surface opposite to the entrance-side surface of the transmission section, and whose transmissivity with respect to the illumination light having passed through the transmission section is one-forth or less that of the transmission section.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.